Intercarrier television receiver circuit



April 18, 1950 R. B. DOME 2,504,662

INTERCARRIER TELEVISION RECEIVER CIRCUIT Filed June 26, 194e l Vig. Z.

61,? His, Attorn ey.

Patented pr. 18, 19511;)

UNITED STATES PATENT OFFICE lNT-RCARRER TELEVISION RECEIVER CIRCUIT Robert B. Dome, Bridgeport,y Conn., assignor to General' Electric Company, a. corporation of NewYork' Application June 26, 1946, Serial-No. 679,341

In the usual. television receiver system, two` intermediate frequencyv channels are employed at some stage following the -rst'v detection of the high` frequency carriers forthe videoand audio signals. In. the.- conventional television circuit,

oneof these intermediate frequency channels hasv a1relatively' broad band widthcharacteristic andl supplies the intermediate frequency signals corresponding to the videol or picture signals. The

other channel has aj comparatively narrow band width characteristic andsupplies: the intermedi-v ate frequency signals corresponding to the audio or sound signals. Each channel, furthermore,.is provided with its own secondv detector and subsequent ampliers. It is apparent that certainA advantages, bothz in operational characteristics and in design. and: cost, may be achieved if. the

signals may be` supplied through a single-intermediate channel. Accordingly, itis another object of my invention to provide a new and improved television receiver circuit which employs a single' ehannelior. translating both audio and video signals.

It is stillanother object or my invention to provide a.- newand. improved television receiver circuitv in which the tuned: circuits. thereof may be' switched by means of push-button switches and satisfactory operation obtained without. the requirement of additional manualadiustment.

One of.r the requirements ofy the circuits employed in the. conventional television receiver is that the oscillator producing the locally generated. oscillations for heterodyning purposes have a. high. frequency stability. Such. requirements not only are diiIicult to meet, but the circuitsnecessary are extremely expensive. Accordingly, it is another objectof my invention to provide a new and.. improved; television. receiver circuit. of the. superheterodynef type which permits a wider` variation in. frequency ofthe .oscillations produced by a local. oscillator includedlin the circuit.

It is a sill further. object` of. my invention to providel a new andv improved televisionreceiver circuit in-which the carrier wave of.- a. video signal employedv as a heterodyni'ng oscillation for audio modulated/waves.

Onev of the featuresof.V my inventionis the use (Cl. FX8-5.8)

ing a broad band pass characteristic for translating. both video and attenuatedI audio modulated waves and heterodyning the video and audio carrier waves. The resultant audio frequency modulated wave is detected in a discriminator circuit,

amplified, and supplied to a loudspeaker.

The features of my invention which I believe to be novel are set forth with particularity in the appended claims. My invention. itself, however, both as to its organization and method of operation, together with further objectsl and advantages thereof, may best be understood by reference to the following description taken in connection with. the. accompanying. drawing in which Fig. 1 represents a block diagram of a. television receiver circuit embodying my. invention; Fig. 2

is a detailed circuit diagram ofv a portion of the receiver circuit of Fig.. 1; and Fig'. 3 illustrates a modification of the circuit of Fig-1.

Referring. to Fig. 1, I have shown an antenna i for receiving television signals. which comprises a picture carrier wavefand an audio or sound carrierv wave. The-frequencies of these carrier waves are. established at vthe transmitters themselves and, through government regulations, are separated in frequency by av frequency of 4.5 megacycles. This 4.5 megacycle frequency difference isA of the order of, but. slightly greater than, theA maximum frequencyA component of a picture signalthat is transmitted. Thus, for example, the video carrier wave frequency may be 51.25 megacycles andthe sound carrier. wave frequency employed by thev same transmitter has a frequency ot 55.75 megacycles.` The antenna l supplies the received carriers to a first detector 2 where they are mixed with: waves or. oscillations generated locally in alocal. oscillator 3.- The output circuits of the first detector 2, therefore, include both a. video intermediate frequency carrier wave anda soundintermediate frequency carrier wave, these two wavesbeing separated. by a frequency difference of 4.5 megacycles. Thus, for example, in the case of the frequencies mentioned above, the local .oscillator may be set to provide waves of` a frequency ofv 67.5 megacycles so that in the output. ofy the rst detector circuits the video of avsingl'e intermediate frequency' channel havse.

intermediate frequency carrier has .a frequency of 16.25V megacycles and the sound intermediate carrier wave has a frequency of. 11.75 megacycles.

The signals at the output of the detector 2 are supplied to an attenuator 4 which comprises a transformer having a pair of windings 5, 6 tuned to theV frequency of the. video intermediate frequency carrier and a. tertiary winding 1 which is tuned bymeans of a-variable capacitance 8 subfe stantially to the frequency of the sound inter mediate frequency carrier wave. The coupling between the winding 'I and the windings 5, E is adjusted so that the attenuator 4 reduces the ratio of the sound to the video waves. Thus, waves of the sound intermediate frequency which are supplied to an intermediate frequency amplier 9 may have an intensity which is only from 1% to 15% the intensity of the video intermediate frequency waves supplied to the amplifier. By such reduction of the intensity of the sound waves relative to the video waves, I reduce the amount of cross modulation between the sound and video signals in a second detector VIII connected to the output of the amplifier 9. Furthermore, in acn cordance with my invention, the amplifier 9 is given' a wide band frequency characteristic so that satisfactory amplification and transmission appears not only the video signals, but also the difference frequency of the video and sound intermediate frequency carrier waves. As previously pointed out, this difference frequency is 4.5 megacycles. It is well known that the conventional television transmitter employs frequency modulation for transmitting sound signais.Y Consequently, the 4.5 megacycle wave, which is present in the output of the second detector I9, is a frequency modulated wave.

The complete video wave and the 4.5 megacycle sound intermediate frequency wave are amplified in a wide band amplifier I I to a level suitable for application to a picture cathode ray tube I2. The picture tube I2 is conventional in form and the details thereof form no part of my present in vention. The picture tube I2 may be supplied with-relatively high voltages for operation from a high voltage supply I3 and may be supplied Withthe usual scanning potentials from a scanning circuit Id which is connected to the video amplifier. The function of the scanning circuit il! is to utilize, in a Well-known manner, the vsynchronizing signalV portion of a received television signal to obtain potentials forV defiecting the cathode ray beamof the tube I2 in horizontal and vertical directions. I

I provide means for removing the audio intermediate frequency carrier waves from thev signais supplied to the picture tube I 2.- This means comprises a lter circuit I5 tuned to 4.5 megacycles, the frequency of the audio intermediate frequency carrier wave present in the output of the amplifier II. The filter I5 is coupled to the input circuit I5 of a limiter-amplifier i?. remainder of the audio circuit includes a third detector I8 which demodulates the frequency modulated audio intermediate frequency wave and supplies, through a volume control I5, audio signals to an audio amplifier 28. The output circuits of the amplifier may be coupled in a conventional manner to a loudspeaker 2i.

In Fig. 2, I have shown the circuit arrangement by which the audio intermediate frequency carrier wave of 4.5 megacycles is removed from the picture tube I2 and utilized for providing audio output voltages to the speaker 2l. `This circuit The comprises a final amplifier 22 of the video :armpitz er circuit, to the control grid 23 of which are supplied both the video signals and the sound modulated 4.5 megacycle carrier from the preceding portions of the receiver circuit. The anode of the amplifier 22 is coupled through the filter I5 to the control electrodeA 2li ofthe picture tube I2. The filter I5 comprises an inductance 25 and a variable capacitance 25 by which the filter circuit may be tuned to resonate at 4.5 megacycles, the carrier frequency of the sound modulated wave. The circuit I 5 thus serves as a final wave trap to keep the 4.5 megacycle wave out of the picture. Coupled to the inductance 25 I circuit I5.

is an inductance 2l which is tuned by means of a capacitance 28 to the frequency of the resonant The elements 2l, 28 are coupled through a grid blocking condenser 29 to the control electrode 30 of a limiter amplifier tube SI. A grid leak 32 is connected between the control electrode 35 and the cathode of the tube BI to pro` vide a path for unidirectional grid current flow.E The anode 33 of the tube 3l is connected to the" primary winding 34 of a discriminator trans-- former 35.

The discriminator circuit may be of any weilknown form and is illustrated as including sec` ondary windings 35, 31 of the transformer 35. The primary winding 34 is, resonated by means of a capacitance 38` at the frequency of the sound 4.5 megacycle carrier Wave. The 'secondary wind-V ings are likewise resonated at the same frequencyI by means of a variable-capacitance 39. v- The adja cent terminals of the-secondary-windings 35, 3i-

are connected to a capacitance 45 and the oppo site terminals of these windings are connected,-

connected to ground, while the opposite terminal` of the resistance dit is connected through a re. sistance 45 and a coupling condenser/ii to the volume control potentiometer i9. The potentiometer I9 includes arvariable contact i8 which is connected to the audio amplifier 2t.- KAn alternative method of deriving audio sig--- nal frequencies fromrthevcontinuous 4.5 megacycle intermediate frequency wave is shown in Fig. 3. This circuit arrangement is essentially the same as that shown in Fig. 2 with theA exception of the connection to the screen electrode* in the limiter amplifier 3 I By virtue of this connection of the screen electrode, the screen elec. trode 49 is employed to control conduction through the limiter amplifier 3l during periods when no picture signals are being received, but in which the cathode ray beam of the tube I2 is being returned to the starting point of its hori zontal sweep, that is, during the horizontal retrace period.

In Fig. 3, the potential for the screen elecv 'are connected in ser-iesacross the secondaryfwind ing of this-transformer; The potential acrossrthe horizontaly deflection coils*` 5U; Sli is likewiseA sup;- plied to the screen electrode wf throughy a re-v sistance 5tA and' acoupling capacitor 55.` A` pair ofv reversely connected diodes 5G; 51 are likewise connected between the screen electrode 49. and ground potentiell The diode. 56 has itsvcathcde connected tothe screen electrode Mlanditsanode connected to-greund and operates: to establish a level for the'- unidirectional voltages supplied'. to. the-screenelectrode $91 The-.anodeof theY diode 5-1 is connected to-thescreen: electrode'. 49' audits cathodey isv connected to ground through; a resist-- ance 58 shuntedf by a capacitance, 5S.

The operation of`thediodes-56, 5ft may best be explained by -referenceto the voltageecurvesi illustratedinfthelower portiorrof Fig; 3. Ifzero un.- directional potential forthescreen` electrode. circuit is illustrated by the horizonta1 line Gil, the horizontalline tlf illustrates the average unidirectionalvoltage level or positivebiasy whichis established'in the screen electrodeicircuit through the operation of the diode 56. Thuethis diode prevents t'he screen electrode ligfrom being driven to large negative potentials; a result which` would be obtained through normal grid rectification by the action of eitherfthediode 5l or the rectification action of thescreen electrode 4S and cathode of the limiter amplier'i. The diode 51 functions to remove thepeaks 52 from the voltage pulse which would otherwise be supplied to the screen electrode 49, clipping the uppersportion of thevoltage Wave-so that/it isprovided with a substantially fiat '001353.-

In` the operation of the circuit ofFig. 3, the continuous 4.5 megacycle wave frequency modu-I lated with desired-audio signals-'is removed from the circuit of the picture tube I2' through the action of the filterv I5 and is supplied to the limiter amplifier 3|'. The screen electrode circuit of the tube/3| functions to prevent transmission ofv the 4.5 megacycle-wave through the device 3|KAV at al1 times,v except: during the horizontal retrace periodof a receivedftelevision signal. Thus, a slightly negativebiasis supplied to the screen electrode 49 during apicture signal period of a received television signal. Positive potential is supplied tothe screen electrode 49 only during theY intervals when horizontal blanking and. synchronizing pulses are` being received. During such intervals, the porticnof'the resultant screen electrode voltage-wave 64 which includes the flat top 63 removes the negative bias from screen electrode 49 and permits,Y transmission of the 4.5 megacycle wave tot the third detectoror discriminator" i8' andultimatelyi to the loudspeaker of the receiver.

One of the advantages of' my: improved audio system for'television receivers is that itk permits theuseof push button switches which arereliable in. operation and do not require an additional manual adjustment' for tuning a.. localoscillator toA an exact. frequency. Such a manual adjustment has` been required heretofore in television rcceiversemploying push button switches for adjusting the. audio circuits as. uncontrollable shifts in. the heterodyning frequencies. occur, which causes` the. sound intermediate. frequency wave to be produced at some frequency other than the one to which the narrow and selective sound intermediate frequency circuit-is tuned..

Another important advantage of my improved system is that, once the circuits have been tuned in the original adjustment of the receiver, the

audio circuits' are*V reliably established and'. re-l quire no- Vfurther attention.y lin'v ai. receiver em.- bodyingvv my: circuit,.1have1foundt that: the fre'- quency of the locali oscillator: 3 may bel changed over a wide range. without.' adversely` affecting the reception of the audio frequencies. Thus, the local oscillator frequency' may-besos detuned that thevideo interm'ediate.v frequency carrierA wave is almost completely exterminated, causing. complete loss-of pictureonthe; picture tube l2,.without seriously. affecting the. quality or volume.A of the sound' translated. to. thet speakerl 2li. Thus; itA is apparent that azrangezo drift of local oscil- Iator frequency considerably' greater: than formerly permitted mayf be tolerated ina receiver which includes myf improvedv audio frequency translating circuits. Furthermore. I have: found that,` due to thelimiting action of. the. limiter amplifier',the:y gain ofthe videoamplier" Il may be varied over a` wide range withoutseriously affecting the sound, level. from ther loudspeaker 2i While have shown and described one particular embodiment `offmy invention, it willzoccur to those skilled in the artgthatmany changes: and modifications may be. made: without departing from my invention, and I.. therefore aim in. the appended claims to cover all! suchl changes. and modifications as fall within the truespirit; and scope of my invention'.

What 'claim asnevv andi desire. to secure by Letters Patent 'ofi theUnitedlStatesfis 1. In a televisionreceiver.` adapted` to receivea picture signal modulated carrien wavev and a continuous` carrier wave modulated by anaudlo frequency signal, said' waves` having' frequencies spaced apart in the frequency spectrum. byy a constant frequency difference greater than but of the order of the maximum frequency component of the picture signal, means forA converting both of said waves to waves of different frequencies, a single channel* adaptedvv to.. translate both of said` converted waves,. meansv for mixing said converted waves to reproduce said' picture signals and to pro-vide anY audio` modulated carrier Wave having aV mean frequency equal to the difference betweenA the frequenciesofv said carrier waves, a`- cathode raytube havingv a-,scanning circuit connected therewith, means" connected to said channel for supplyingv picture--signalsto said tube, means for removing. said difference irequency fromsaid' channel andi for reproducing said' audiosignals from said' difference frequency wave, said last means including an electron discharge' device having-ga screen electrode, means connected to said scanning circuit f'or rectifying current therein, and means supplying said rectified current to said screen electrode with a polarity such that said device is-A rendered non-conductive during'peri'ods when apicturesignal is being received and conductive during periods when a blanking pulseiisbeing received.

2i. In a. televisionzreceiver', means. to receive a television signal band' including a.. videovv carrier having successive picture and retrace modulation intervals and ai relate'dfaudio frequency modulated carrier, said carriersibeing separatedzin frequency bya constan-t"differencezfrequency; .a singlechannel connectedl totranslate. said. carriers, means for mixing said carriersto produce.'l a. video vwave andA anaudio modulated carriery wave having a frequencyV equal to saidY difference frequency, means for developingl a. keying wave from said video wave, means utilizing.V saidV keying waveA to select-those portions of: said: difference frequency carrier.` wave: whicln occur. duringsaid retrace intervals, a frequency fdlscriminator tuned to said difference frequency, means for supplying-said selected portions -of said difference frequency carrier wave to said discriminator, and a reproducing device responsive tothe output from said discriminator.

3. In a television receiver, means to receive a television signal band including a video carrier having successive picture .and retrace modulation intervals and a related audio frequency modu lated carrier, said carriers being separated in frequency by a Vconstant difference frequency, a single channel connected to translate said carriers, means for mixing said carriers to produce a video wave and an audio modulated carrier wavehaving a frequency equal to said difference frequency, means for delevoping a keying wave from said video Wave, a frequency discriminator tuned to said dierence frequency, means including an electron discharge device for supplying said difference frequency carrier Wave to said discriminator, means utilizing said keying wave for rendering said device conductive only during said retrace intervals thereby to supply said difference frequency carrier wave to said discriminator only duringr said retrace intervals, and a reproducing device responsive to the output from said discriminator.

4. In a television receiver, means to receive a television signal band including a video carrier having successive picture and retrace modulation intervals and a related audio modulated carrier, said carriers being separated in frequency by a constant difference frequency, means for converting both said carriers to Waves of different frequencies, a single channel adapted to translate both of said converted waves, means for mixing said converted Waves to reproduce said video modulations and to provide an audio modulated carrier wave having a frequency equal to said difference frequency, means for developing a keying wave from said video modulations, means utilizing said keying Wave to select those portions of said difference frequency carrier wave which occur during said retrace intervals, a Vfrequency discriminator tuned to said difference frequency, means for supplying said selected portions of said differencerequency carrier Wave to said discriminator, and a reproducing device supplied with the output from said4 discriminator.

5. In a television receiv er,rmeans to receive a television signal bandincluding a video carrier having successive picture and retrace modulation intervals and a related audio modulated carrier, said carriers being separated in frequency by a constant difference frequency, means for converting both said carriers to Waves of different fref quencies, a single channel adapted to translate both of said converted waves, means for mixing said converted waves to reproduce said video modulations and to provide an audio modulated carrier wave having a frequency equal to said difference frequency, means for developing a keying wave from said video modulations, a frequency discriminator tuned to said difference frequency, means including an electron discharge device for connecting said difference frequency carrier wave to said discriminator, means utilizing said keying Wave for rendering said device conductive only during said retrace intervals thereby to supply said difference frequency carrier wave to said discriminator only during said retrace intervals, and a reproducing device responsive to the output from said discriminator.

6. In a television receiver, means to receiveV a television signal band including a video carrier having successive picture and retrace modulation intervals, and a related audio modulated carrier, said carriers being separated in frequency by a constantV difference frequency, means for converting both said carriers to waves ofdifferent frequencies, a single channel adapted Ato trans-l ate both of said converted Waves, means, for mixlng said lconverted waves toreproduce said video modulations and to provide an audio modulated carrier Wave having a frequency equal to said difference frequency, means for developing a keying Wave from said video modulations, means for reproducing said audio signals from said dilerence frequency carrier wave, said last named means including an electron discharge device having a screen electrode, `and means to supply said keying Wave to said screen electrode thereby to render said device conductive only during said retrace intervals.

7. In a television receiver, means to receive a television signal band. including a video carrier having successive picture and retrace modula-V tion intervals and a related audio modulated carrier, said carriers being separated in frequency by a constant difference frequency, means for converting both of said carriers to waves of different frequencies, a singlechannel adapted to translate both of said converted Waves, means for mixing said converted Waves to reproduce said video modulations andy to provide an audio modulated carrier Wave having a frequency equalto said difference frequency, means for developing` a keying wave lfrom said video modulations, said keying wave comprising pulses which occur during said retrace intervals, an electron discharge device having first and second control electrodes, means for supplying said difference frequency carrier wave to said i'lrst control electrode, means for supplying said keying pulses to said second electrode with a polarity to render said device conductive during said keying pulses thereby to select those portions of said difference frequency carrier wave which occur during said keying pulses, and means for reproducing said audio signals from the selected portions of `said difference frequencygcarrier wave. Y

ROBERT B. DOME. l,

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,457,447l Mills June 5, 1923 1,465,961 Alexanderson Aug. 28, 1923 1,495,479 Farrington May 27, 1924 1,629,685 Ditcham May 24, 1927 1,681,564 Wright Aug. 21, 1928 1,735,134 Schroter Nov. 12, 1929 1,797,317 Brand Mar. 24, 1931 2,056,607 Holmes Oct. 6, 1936 2,118,610 Koch May 24, 1938 2,146,876 zworykin Feb. 14, 1939 2,164,745 Kentner July 4, 1939 2,268,001 Franholz Dec. 30, 1941 2,350,902 Kallman June 6, 1944 2,403,957 Seeley July 16, 1946 2,448,908 Parker Sept. 7, 1948 OTHER REFERENCES (COPY Certificate of Correction Patent No. 2,504,662 April 18, 1950 ROBERT B. DOME It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows:

Column 6, line 32, strike out adapted and insert instead a comma and the Word ,means;

and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Office. Signed and sealed this 26th day of September, A. D. 1950.

THOMAS F. MURPHY,

Assistant ommssioner of Patents. 

